This invention relates generally to spent nuclear fuel storage and more particularly to systems and methods for absorbing neutrons in a nuclear fuel storage rack.
Nuclear power plants store their spent nuclear fuel in spent fuel storage racks located in on site storage pools. Such racks may be, for example, a Flux Trap Nuclear Fuel Storage Rack 10 as depicted in FIG. 1 or an Eggcrate Nuclear Fuel Storage Rack 100 as depicted in FIG. 2. These racks include neutron absorber material 20 (FIGS. 1-2 and 17-18) to control the re-activity state of the fuel/rack system which is located between a stainless steel inner cell wall 7 and a stainless steel wrapper 9. Specifically, such absorbers are utilized to neutronically decouple adjacent fuel cell assemblies by maintaining a subcritical condition. In a flux trap storage rack (FIG. 1), two plates of absorber material 20 separate each storage cell. For example, storage cell 30 and storage cell 35 are separated by plates 32 and 33. In an eggcrate system (FIG. 2), one plate of absorber material separates each storage cell. For example, cell 50 and cell 60 are separated by a plate 55.
This absorber material may be a boron bearing compound in a metal or polymer matrix which may degrade over time as it absorbs neutrons from the spent fuel. For example, the absorber material may be formed of a polymer matrix containing boron carbide (e.g., a BORAFLEX type polymer) and the absorber material may degrade and decrease over a period of several years. Also, charge enrichments for reload fuel for nuclear reactors are generally increasing relative to previous change enrichments and can be as high as 5.0 w/o U-235. The degradation of the absorber material and the use of higher enrichments require replacement of the spent fuel storage racks with racks containing additional absorber material or the insertion of additional absorber material into existing spent nuclear fuel storage racks to control the re-activity state of the fuel/rack system.
Thus, there is a need for a systems and methods for absorbing neutrons in spent nuclear fuel storage racks and particularly systems and methods for restoring neutron absorbing material into spent nuclear fuel storage racks.
The present invention provides, in a first aspect, a neutron absorber system for a nuclear fuel storage rack. The system includes a neutron absorber, which is adapted to attach to a plurality of cell walls of a cell of the nuclear fuel storage rack. The neutron absorber is adapted to elastically deform to cause it to be attached to the plurality of cell walls.
The present invention provides, in a second aspect, a method of inserting a neutron absorber into a nuclear fuel storage rack. The method includes applying at least one stress to the neutron absorber to cause the absorber to deform and releasing the stress to cause the neutron absorber to attach to the plurality of cell walls of a cell of the nuclear fuel storage rack.
The present invention provides, in a third aspect, a system for inserting a neutron absorber into a nuclear fuel storage rack. The system includes means for applying at least one stress to the neutron absorber wherein the neutron absorber is adapted to elastically deform. The system further includes means for releasing the at least one stress to cause the neutron absorber to attach to the plurality of cell walls of a cell of the nuclear fuel storage rack.